git-stash: allow more descriptive reminder message when saving

git-stash: allow more descriptive reminder message when saving

This allows you to say:

	$ git stash starting to implement X

while creating a stash, and the resulting "stash list entry
would read as:

	$ git stash list
        stash@{0}: On master: starting to implement X

instead of the default message which talks about the commit the
stash happens to be based on (hence does not have much to do
with what the stashed change is trying to do).

Signed-off-by: Junio C Hamano <gitster@pobox.com>